An office machine such as a printer or a scanner is widely used in the office. For achieving diversified functions and integrating more functions, the office machine is usually equipped with a post-processing device (e.g. a stapler). By the stapler, a plurality of documents outputted from the printer or the scanner can be automatically stapled in order to enhance the working efficiency. The operations of the post-processing device (e.g. a stapler) will be illustrated as follows. Firstly, the documents are placed on a sheet placement tray. Then, the edges of these documents are aligned with each other by a sheet alignment device. Then, a stapling operation is performed by the stapler, so that the same parts of these documents are combined together.
Please refer to FIGS. 1A and 1B. FIG. 1A is a schematic planar view illustrating a conventional sheet alignment device. FIG. 1B is a schematic cross-sectional view illustrating the conventional sheet alignment device. The conventional sheet alignment device as shown in FIGS. 1A and 1B is disclosed in for example U.S. Pat. No. 7,134,659.
The sheet alignment device 1 comprises a first alignment arm 11, a second alignment arm 12, a sensor 13 and a sheet placement tray 14. The first alignment arm 11 comprises a first platform 11a, a first sidewall 11b and a protrusion structure 11c. The second alignment arm 12 comprises a second platform 12a and a second sidewall 12b. 
The first sidewall 11b of the first alignment arm 11 is extended from an end of the first platform 11a and perpendicular to the first platform 11a. The protrusion structure 11c is extended from the first sidewall 11b. The second sidewall 12b of the second alignment arm 12 is extended from an end of the second platform 12a and perpendicular to the second platform 12a. The sensor 13 is arranged beside the protrusion structure 11c. The sheet placement tray 14 is disposed under the first alignment arm 11 and the second alignment arm 12. Moreover, the first platform 11a of the first alignment arm 11 and the second platform 12a of the second alignment arm 12 are coplanar.
Hereinafter, the operations of the conventional sheet alignment device 1 will be illustrated with reference to FIGS. 1A and 1B. In a case that no sheet is introduced to the sheet alignment device 1, the first alignment arm 11 and the second alignment arm 12 are respectively located at a first home positional and a second home position a2. Meanwhile, the sensor 13 is interrupted by the protrusion structure 11c of the first alignment arm 11, and thus a first sensing signal is issued by the sensor 13.
After the sheet is introduced into the sheet alignment device 1, the power device (not shown) is controlled by a controller (not shown) to drive both of the first alignment arm 11 and the second alignment arm 12 to be inwardly moved by a preset distance d1. Consequently, the first alignment arm 11 and the second alignment arm 12 are moved to a first standby position b1 and a second standby position b2 so as to carry the sheet S. Meanwhile, the protrusion structure 11c of the first alignment arm 11 is distant from the sensor 13, and thus a second sensing signal is issued by the sensor 13.
When the sheet-aligning action starts, according to the size of the sheet S and under control of the controller (not shown), the first alignment arm 11 is moved toward the second alignment arm 12 by an alignment distance d2 to an alignment position c. Meanwhile, the edge of the sheet S is nestled against the second sidewall 12b. 
If another sheet is ready to be introduced into the sheet alignment device 1, the power device (not shown) is controlled by the controller (not shown) to drive the first alignment arm 11 to be distant from the second alignment arm 12 by the alignment distance d2. Consequently, the first alignment arm 11 is returned to the first standby position b1. Similarly, the first alignment arm 11 is moved between the first standby position b1 and the alignment position c in a reciprocating manner until the rest of the sheets implement the sheet-aligning actions.
After the sheet-aligning actions of all sheets have been implemented, a stapling operation is performed by a post-processing device (e.g. a stapler), so that the same parts of these sheets are combined together. Then, the power device (not shown) is controlled by the controller (not shown) to drive both of the first alignment arm 11 and the second alignment arm 12 to be outwardly moved until the sensor 13 is interrupted by the protrusion structure 11c of the first alignment arm 11 again and the first sensing signal is issued by the sensor 13. That is, the first alignment arm 11 and the second alignment arm 12 are respectively returned to the first home position a1 and the second home position a2. Afterwards, the stapled sheets are introduced to the sheet placement tray 14 and ejected to the outer portion of the office machine.
From the above discussions, a plurality of sheets are aligned with each other by moving the first alignment arm 11 and the second alignment arm 12 of the conventional sheet alignment device 1. In addition, the use of the sensor 13 can detect the positions of the first alignment arm 11 and the second alignment arm 12.
The conventional sheet alignment device, however, still has some drawbacks. For example, during the process of performing the sheet-aligning actions of the sheets, the power device (not shown) drives the first alignment arm 11 to be moved between the first standby position b1 and the alignment position c in a reciprocating manner. In addition, the sensor 13 is only able to detect whether the first alignment arm 11 and the second alignment arm 12 are respectively located at the first home position a1 and the second home position a2. On the other hand, the sensor 13 fails to judge whether the first alignment arm 11 is really moved to the first standby position b1. If the first alignment arm 11 is moved by a distance shorter than the alignment distance d2 after the sheet-aligning action of the first sheet is completed, the first alignment arm 11 fails to be moved to the actual first standby position b1. Under this circumstance, the real position of the first alignment arm 11 is separated from the first standby position b1 by an error distance. After the sheet-aligning action of the second sheet is completed, the edges of the second sheet and the first sheet are separated from each other by the error distance. In other words, the second sheet fails to be precisely aligned with the first sheet. If the error distance is generated again during the subsequent sheet-aligning actions are performed, the total error distances will be largely increased. After the sheet-aligning actions of all sheets have been implemented and the sheets are combined together, the stapled parts of these sheets are not at the same position.
Therefore, there is a need of providing an improved sheet alignment device so as to obviate the drawbacks encountered from the prior art.